Prominent among the selective non-catalytic reduction (SNCR) processes are those disclosed for example by Lyon in U.S. Pat. No. 3,900,554 and by Arand et al in U.S. Pat. Nos. 4,208,386 and 4,325,924. Briefly, these patents disclose that ammonia (Lyon) and urea (Arand et al) can be injected into hot combustion gases to selectively react with NO.sub.x and reduce it to diatomic nitrogen and water.
The SNCR process described by Lyon in U.S. Pat. No. 3,900,554 reduces the concentration of nitrogen monoxide (NO) in combustion gases. Lyon discloses injecting ammonia or certain ammonia precursors or their aqueous solutions into an oxygen-rich waste gas for selective reaction with the nitrogen monoxide at a temperature in the range of from 870.degree. to 1100.degree. C. In this process, it is important that the temperature of the combustion effluent lie within a narrow "temperature window" during the contact with the gaseous ammonia. The limiting values of the window can be reduced by the addition of certain substances. Distribution of the ammonia within the combustion effluent is critical to achieving maximum utilization of the ammonia and reduction of NO within the defined temperature window. Ineffective utilization will increase costs and cause other problems associated with ammonia discharge.
Arand et al disclose in U.S. Pat. No. 4,208,386 that urea can be added alone or in solution to oxygen-rich effluents in a temperature range from 700.degree. to 1100.degree. C. Any urea which fails to react with NO.sub.x within the temperature window is, nonetheless chemically transformed by heat and some, during cooling, results in ammonia formation. Again, here, as with the Lyon process, distribution is critical to selective reduction and, therefore, to economic operation and avoidance of the problems associated with ammonia discharge and fouling.
Similarly, in U.S. Pat. No. 4,325,924, Arand et al describe an SNCR process utilizing urea in fuel-rich combustion effluents. Effluents of this type can be generated by staged combustion, which can lead to the formation of high levels of carbonaceous pollutants. Again, distribution is critical and, if ineffective, can have adverse economic as well as environmental impact.
A number of other disclosures in the field of SNCR suggest improvements over the aforementioned processes. For example, in U.S. Pat. No. 4,992,249, Bowers discloses that if droplet size is increased and urea concentration is decreased, good results can be achieved in oxygen-rich effluents at higher temperatures than disclosed by Arand et al. Distribution, however, remains critical here, and there is a need to better control droplet size.
In a further modification, Bowers discloses in U.S. Pat. No. 4,719,092 that an additional material, an oxygen-containing hydrocarbon, can be injected together with an aqueous urea solution to reduce residual ammonia concentration in the effluent. Despite the added material, distribution and droplet size distribution remain important.
In an effort to achieve better distribution by injection, DeVita describes an injector in U.S. Pat. No. 4,915,036 which shows good distribution of injected fluids while the danger of clogging is minimized. This specification discloses the need for good distribution of chemicals and enables improving it where boiler geometry permits. Similarly, in U.S. Pat. No. 4,842,834, Burton describes an injector which, while effective in many combustor configurations, could benefit from the provision of an additive for the aqueous solutions employed which could improve droplet size distribution. And, in WO 91/17814, Chawla et al describe a nozzle which enables injection of a two-phase mixture of air and aqueous NO.sub.x -reducing composition into an effluent at sonic velocity to achieve an improved distribution of particle sizes, but could also benefit from such an additive.
There remains a present need for a process to effectively minimize combustion-generated pollutants, such as nitrogen oxides, while simultaneously minimizing secondary pollutants, such as carbon monoxide and ammonia, in the final effluent by achieving more uniform distribution of pollutant reducing agents at the effective temperature ranges for the chemicals concerned, especially in effluent passages having geometries and load-determined temperature profile characteristics which adversely impact distribution.